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Register a new userEvent-by-event study of prompt neutrons from 239Pu(n,f)
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Employing a recently developed Monte Carlo model, we study the fission of 240Pu induced by neutrons with energies from thermal to just below the threshold for second chance fission. Current measurements of the mean number of prompt neutrons emitted in fission, together with less accurate measurements of the neutron energy spectra, place remarkably fine constraints on predictions of microscopic calculations. In particular, the total excitation energy of the nascent fragments must be specified to within 1 MeV to avoid disagreement with measurements of the mean neutron multiplicity. The combination of the Monte Carlo fission model with a statistical likelihood analysis also presents a powerful tool for the evaluation of fission neutron data. Of particular importance is the fission spectrum, which plays a key role in determining reactor criticality. We show that our approach can be used to develop an estimate of the fission spectrum with uncertainties several times smaller than current experimental uncertainties for outgoing neutron energies up to 2 MeV.
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Earlier studies of 239Pu(n, f) have been extended to incident neutron energies up to 20 MeV within the framework of the event-by-event fission model FREYA, into which we have incorporated multichance fission and pre-equilibrium neutron emission. The main parameters controlling prompt fission neutron evaporation have been identified and the prompt fission neutron spectrum has been analyzed by fitting those parameters to the average neutron multiplicity nubar from ENDF-B/VII.0, including the energy-energy correlations in nubar(E) obtained by fitting to the experimental nubar data used in the ENDF-B/VII.0 evaluation. We present our results, discuss relevant tests of this new evaluation, and describe possible further improvements.
Prompt fission neutron spectra from $^{239}$Pu($n,f$) were measured for incident neutron energies from $0.7$ to $700,$MeV at the Weapons Neutron Research facility (WNR) of the Los Alamos Neutron Science Center. A newly designed high-efficiency fission chamber was coupled to the highly segmented Chi-Nu array to detect neutrons emitted in fission events. The double time-of-flight technique was used to deduce the incident-neutron energies from the spallation target and the outgoing-neutron energies from the fission chamber. Prompt fission neutron spectra (PFNS) were measured with respect to $^{252}$Cf spontaneous fission down to $200,$keV and up to about $12,$MeV for all the incident neutron energies with typical uncertainties well below $2%$ up to about $10,$MeV outgoing-neutron energy. The general trend of PFNS is well reproduced by JEFF3.3 and ENDF-BVIII.0 evaluations. Discrepancies were however observed for the low-energy part of the spectra, where evaluations overestimate the number of emitted neutrons. Neutron multiplicities and average kinetic energies as a function of incident-neutron energy are obtained experimentally with reported uncertainties below $0.4%$. Neutron multiplicities disagree with some older datasets above $6,$ MeV, indicating the need of using a high-efficiency fission detector, which does not bias the data. The measured mean kinetic energies agree with the most recent data. Evaluations fairly reproduce the trend, but fail to reproduce the experimental values within their uncertainties.
The increased interest in more exclusive fission observables has demanded more detailed models. We present here a new computational model, FREYA, that aims to meet this need by producing large samples of complete fission events from which any observable of interest can then be extracted consistently, including arbitrary correlations. The various model assumptions are described and the potential utility of the model is illustrated by means of several novel correlation observables.
In a noncentral heavy-ion collision, the two colliding nuclei have finite angular momentum in the direction perpendicular to the reaction plane. After the collision, a fraction of the total angular momentum is retained in the produced hot quark-gluon matter and is manifested in the form of fluid shear. Such fluid shear creates finite flow vorticity. We study some features of such generated vorticity, including its strength, beam energy dependence, centrality dependence, and spatial distribution.
Event-by-event viscous hydrodynamics is combined with heavy quark energy loss models to compute heavy flavor flow cumulants $v_2{2}$, $v_3{2}$, and $v_2{4}$ as well as the nuclear modification factors of $D^0$ and $B^0$ mesons in PbPb collisions at 2.76 TeV. Our results indicate that bottom quarks can flow as much as charm quarks in the $p_T$ range 8--30 GeV.
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